1. Field of the Invention
This invention relates to a method of driving data lines in a liquid crystal display (LCD), and more particularly to a data line driving method wherein the data lines are pre-charged using sampling switch control signals of the data lines to thereby be initialized and a liquid crystal display device employing the method.
2. Description of the Related Art
A liquid crystal display (LCD) is a flat panel display device having the benefits of a small size, a thin thickness and low power consumption. Such an LCD has been used for a notebook personal computer (PC), office automation equipment and audio/video equipment, etc. Particularly, an LCD of the active matrix type makes use of a thin film transistor (TFT) as a switching device to display a dynamic image. Recently, there has been actively made a study as to a poly-silicon TFT capable of integrating more peripheral driving circuits than the existent amorphous silicon TFT.
As shown in FIG. 1, such an LCD includes a pixel array 10 having pixels (or picture elements) arranged at intersections between Nn data lines DL11, DL12, . . . , DLNn and m gate lines GL1, GL2, . . . , GLm in a matrix pattern, and a sampling switch part 20 installed between N video bus lines VL1, VL2, . . . , VLN and the Nn data lines DL11, DL12, . . . , DLNn to apply video signals Video1, Video2, . . . , VideoN to the data lines DL11, DL12, . . . , DLNn. The sampling switch part 20 applies the N video signals Video1, Video2, . . . , VideoN to the Nn data lines DL11, DL12, . . . , DLNn to reduce the number of video bus lines VL1, VL2, . . . , VLN. This sampling switch part 20 includes N demultiplexors DMX1, . . . , DMXN connected between any one line of the N video bus lines VL1, VL2, . . . , VLN and n data lines. Each of the demultiplexors DMX1, . . . , DMXN includes n TFTs.
Each of TFTs T11, T12, . . . , TNn is turned on in accordance with control signals φ1, φ2, . . . , φn to apply video signals coupled via demultiplexor input lines DIL1, . . . , DILN connected to any one line of the N video bus lines VL1, VL2, . . . , VLN to the data lines. The control signals φ1, φ2, . . . , φn applied to gate terminals of the TFTs T11, T12, . . . , TNn are generated by a demultiplexor control signal generator 22. As shown in FIG. 2, each of the control signals φ1, φ2, . . . , φn is synchronized with the video signal during one horizontal synchronizing signal interval 1H to be changed sequentially into a high logic level. Each TFT T11, T12, . . . , TNn is sequentially turned on in response to the control signals φ1, φ2, . . . , φn to sequentially apply the corresponding video signal to the data lines DL11, DL12, . . . , DLNn.
Meanwhile, in order to improve picture quality, data voltages having the contrary polarity with respect to each other are applied to the adjacent data lines DL11, DL12, . . . , DLNn. Thus, the pixels are charged or discharged to a different voltage level to generate a voltage difference. The voltage difference in the pixels cause a color signal difference and a brightness difference between the adjacent pixels to deteriorate the picture quality. For instance, as shown in FIG. 3, red pixel connected to the first data line DL11 and the adjacent green pixel are supplied with 6V and −3V respectively whereas blue pixel connected to the third data line DL13 is supplied with 6V. In this case, the pixels are charged to 5.8V, −2.8V, and 5.9V respectively, by their coupling with the adjacent pixels, so that a desired color signal and brightness can not be obtained. Also, if data voltages with opposite polarity are applied to the data lines DL11, DL12, . . . , DLNn, then the power consumption is increased because each line has a voltage difference as large as a voltage variation difference between the data lines or the pixels.
In order to overcome this problem, as shown in FIG. 1, the LCD includes a pre-charging switch part 30 for charging the data lines DL11, DL12, . . . , DLNn to a certain intermediate level. The pre-charging switch part 30 charges all of the data lines DL11, DL12, . . . , DLNn into a pre-charging signal Vpc before application of the video signals to initialize the data lines DL11, DL12, . . . , DLNn. The pre-charging signal Vpc is supplied from a pre-charge line PCL provided at the lower end of the pixel array 10. The pre-charging switch part 30 includes Nn TFTs CT11, CT12, . . . , CTNn connected between the data lines DL11, DL12, . . . , DLNn and the pre-charge line PCL. Each of the TFTs CT11, CT12, . . . , CTNn is turned on in accordance with a pre-charge control signal Pre-EN to connect all of the data lines DL11, DL12, . . . , DLNn to the pre-charge line PCL. As seen from FIG. 2, the pre-charge control signal Pre-EN is generated from the control signal generator 32 before the video signals are applied to the data lines DL11, DL12, . . . , DLNn.
If the data lines DL11, DL12, . . . , DLNn are charged into an intermediate voltage before data is supplied, then the voltage variation is reduced by one-half during the charge or discharge of the data lines or the pixels, so that coupling between the data lines or the pixels is reduced to improve the picture quality characteristic. The power consumption is reduced as much as the voltage variation width is reduced due to the pre-charge. Also, a swing width of an output signal of a data driver (not shown) for applying video signals to video bus lines VL1, VL2, . . . , VLN is reduced by one-half, so that the charge time of the data lines or the pixels is reduced.
On the other hand, as shown in FIG. 4, the pre-charge line PCL may be provided at the upper portion of the pixel array 10. In this case, pre-charging TFTs CT11, CT12, . . . , CTNn are provided between the pre-charge line PCL and the demultiplexor TFTs T11, T12, . . . , TNn.
However, the conventional pre-charging switch part 30 has a drawback in that, since it requires the additional TFTs CT11, CT12, . . . , CTNn and the pre-charge control signal generator 32, the effective display area of the display panel is reduced. Also, it has a drawback in that, since the pre-charge control signal in the prior art requires a level shifter to produce a high voltage pulse of 15 to 20 Vpp, its manufacturing cost rises. Moreover, the conventional pre-charge switch part 30 has a problem in that, since a leakage current is generated by the TFTs CT11, CT12, . . . , CTNn to cause a voltage variation in the data lines or the pixels, the picture quality is deteriorated.